JWs say that "the light gets brighter" because the understanding of their leadership is not perfect (not completely correct) and is constantly subject to revision.
One might ask, if the teachings are Bible Based ™ , how can they be subject to revision? Because the Bible doesn't give explicit directives/commentary on every question of life we may encounter. Such matters must be inferred from what the Bible does comment on. This is interpretation. Such interpretation is done by humans, who despite thier most honest attempts could very well get things wrong.
So why aren't present teachings which are not explicit in the Bible but inferred through fallible human interpretation qualified as such when they are put forth? For instance, the bible says nothing about blood transfusions. Why does the Society ban transfusions of four particular components of blood without qualifying this stance as fallible--particularly since this is a life and death issue?
This is because the implications of the NEW LIGHT doctrine are not meant to apply to present teaching. The doctrine is only meant to apply to past teaching.
Present teaching falls under a different doctrine, the doctrine of the Faithful Discreet Slave. When a JW views present teaching he must acknowledge that it comes straight from God via the FDS and that for all intents and purposes it is just as if the Bible stated the teaching explicitly. IT IS INFALLIBLE.
So there you have it, folks. A classic example of Orwellian doublethink: What the WTS says is fallible. What the WTS says is infallible.